Provides a method for use by the Heap processing routines. If a value argument is provided, it will be saved. The new saved value is always returned. If no value argument is provided, the old saved value is returned.

The Heap processing routines use this method to map an element into its internal structure. This is needed to support the Heap methods that affect elements that are not are the top of the heap - decrease_key and delete.

The Heap processing routines will ensure that this value is undef when this elem is removed from a heap, and is not undef after it is inserted into a heap. This means that you can check whether an element is currently contained within a heap or not. (It cannot be used to determine which heap an element is contained in, if you have multiple heaps. Keeping that information accurate would make the operation of merging two heaps into a single one take longer - it would have to traverse all of the elements in the merged heap to update them; for Binomial and Fibonacci heaps that would turn an O(1) operation into an O(n) one.)

A routine to compare two elements. It must return a negative value if this element should go higher on the heap than $elem2, 0 if they are equal, or a positive value if this element should go lower on the heap than $elem2. Just as with sort, the Perl operators <=> and cmp cause the smaller value to be returned first; similarly you can negate the meaning to reverse the order - causing the heap to always return the largest element instead of the smallest.

This class can be inherited to provide an oject with the ability to be heaped. If the object is implemented as a hash, and if it can deal with a key of heap, leaving it unchanged for use by the heap routines, then the following implementation will work.